Using competitive native plants to achieve grassland restoration goals

Background/Question/Methods

Successful ecosystem restoration involves increasing populations of native species, restoring ecosystem functions, and mitigating threats to community stability.   In California grasslands, re-invasion by non-native plants is a key threat to restoration.  Through a field-based experiment in coastal grasslands, we explored whether restoration actions that increase densities of known native competitors could reduce vulnerability to ongoing invasion.  We focused on interactions between yellow starthistle (Centaurea solstitalis), a noxious invasive annual forb currently spreading across the Western U.S., and two native tarweeds, Hemizonia congesta and Madia elegans.  Past studies indicate that H. congesta and starthistle use resources similarly in time and space, and that declines in H. congesta abundance correlate with greater starthistle invasion.  We planted varying abundances of one or both tarweed species into grassland plots, where the previous year’s litter was either cut and cleared, or cut and retained. 

Results/Conclusions

We found that starthistle resistance was similarly achieved via two different strategies:  (1) retaining cut litter in plots, or (2) removing litter and adding a high density of tarweeds.  While neither strategy reduced starthistle to below levels found in undisturbed grasslands, retaining cut litter was slightly more successful, and adding native tarweeds to litter retention plots did not improve overall invasion resistance.  In plots cleared of litter, the native seeding mixture’s density and to a lesser extent composition, affected starthistle biomass and flower production, possibly due to more complete resource utilization by higher densities of competitive plants and richer seed mixtures.  The two restoration strategies, while similarly affecting total starthistle re-invasion, resulted in different final grassland composition.  Seed addition plots contained a greater density of native species, largely because of the increase in tarweed abundances.  Our results support the hypothesis that inclusion of competitive species in restoration actions can contribute to multiple restoration goals:  both increasing numbers of native species, and while not eliminating non-native species, decreasing re-invasion success.